Extruder types are known which, because of their type of construction, do not permit the degasifying of a liquified material along the extrusion unit. An important representative of this extruder type is the so-called "planetary roller extruder", as known, for example, from German Patent Document DE-PS 31 33 647. In the case of these extruders, a degasifying of the delivered medium can take place only after it leaves the extrusion unit.
Although conventional degasifying extruders have one or several degasifying openings-in the cylinder, the effectiveness of the degasifying of the liquified material is limited considerably because of the small achievable surface of the liquified material.
In order to meet the requirement of an intensive degasifying of the liquified material, two-stage extrusion systems of a cascade construction were suggested, in which case a degasifying shaft was inserted between the extrusion stages and a vacuum pump was connected.
Cascade systems of this type have the disadvantage of a large processing length which requires correspondingly high expenditures for the thermal stabilization of the delivered medium. In addition, a large processing length requires the corresponding space as well as large amounts of driving and heating energy.
From U.S. Pat. No. 3,817,668, it is known to deliver by means of a gear pump predominantly low-viscous thermoplastic polymer liquified material from a receptacle which is subjected to a vacuum with respect to the ambient pressure, for example, from a reactor bulb.
Because of their constructive design and the limited usage spectrum (viscosity), these pumps, which have been known from the synthesis of polymers, were rarely used in the extrusion of plastics.
Within the scope of an increasing environmental awareness, there is now, on the one hand, a growing requirement for recycling post consumer plastic waste; on the other hand, an increasing amount of thermally sensitive thermoplastic material and particularly PVC-materials are to be processed.
The present invention relates to the processing of thermoplastic post consumer waste and of PVC.
Within the scope of a first aspect, the invention has the object of further developing a process or a treatment stage of the above-noted type in such a manner that it can be used for thermoplastic post consumer wastes or for PVC.
In the case of a process of the above-noted type, this is achieved according to certain preferred embodiments of the invention by providing an arrangement including a gear pump fed by at least one billet-like shape of the material of a given geometry. In especially advantageous contemplated embodiments of the invention a treatment stage is provided with a degasifying shaft which is subjected to a vacuum and with a gear pump for the build-up of the delivery pressure of the material, which gear pump is connected behind the degasifying shaft and can be charged with a vacuum with respect to the ambient pressure, wherein the gear pump is fed by means of a multiple extrusion die for treatment of PVC or of thermoplastic plastic wastes of a varying composition, which are contaminated to a varying degree.
A reason that the invention is advantageous is that it was found that, for the processing according to the invention, on the one hand, of thermoplastic post consumer wastes, their composition can be varied within a very wide range and they may be contaminated to different degrees; and, on the other hand, for the processing of PVC, the feeding is required of the provided gear pumps by means of a billet of the material of a given geometry, preferably by means of a multiple billet. When processing PVC, a delivery of the gear pump that is free of disturbances will exist only when a constant given geometry of the billet is ensured.
In the case of the above-mentioned wastes, solvent portions of printing dyes and residual moisture portions of any type must be removed in order to obtain qualitatively useful secondary products.
In order to therefore utilize for the degasifying an achievable surface of the material that is as large as possible, it is suggested according to preferred embodiments of the invention to form the above-mentioned billet-like shape or preferably the multiple billet before the degasifying and to therefore guide it through the degasifying shaft to the inlet of the provided gear pump.
For the intensive degasifying of the material, a surface of the material is required which is as large as possible relative to the volume of the same.
When the approach according to the present invention is used, it is easily possible for the material to be processed to keep the process length short compared with the cascade systems and to connect behind an extruder a degasifying shaft and to connect behind the latter a gear pump that can be charged from a vacuum.
From Swiss Patent Document CH-PS-573 053 and U.S. Pat. No. Document U.S. Pat. No. 3,746,481, gear pumps are known which are designed for being charged with a vacuum with respect to ambient pressure. These pumps therefore make it possible to be operated in a completely filled manner even when charged from the vacuum and to thus furnish the constant delivery pressure which is typical of gear pumps. However, these pumps are not suitable for the delivery of very contaminated thermoplastic material, particularly on the basis of rotor shaft seals, normally by means of axial face seals or gland packings.
Under another aspect, the present invention therefore has the object of further developing a pump of the latter type, that is, a pump which is suitable for the delivery of highly viscous media, such as plastic liquified material in general, specifically from a vacuum with respect to ambient pressure, and which has a charging hopper which is situated above the gearwheels and tapers toward the gearwheels, in such a manner that this pump can be used for the delivery of the above-mentioned contaminated thermoplastic material of a varying composition, and also for the delivery of PVC.
This object is achieved according to preferred embodiments by providing at treatment stage, a treatment stage for thermoplastic material with a degasifying shaft which is subjected to a vacuum and with a gear pump for the build-up of the delivery pressure of the material, which gear pump is connected behind the degasifying shaft and can be charged with a vacuum with respect to the ambient pressure, wherein the gear pump is fed by means of a multiple extrusion die for treatment of PVC or of thermoplastic plastic wastes of a varying composition, which are contaminated to a varying degree
It is known from European Patent Document EP-A-0 189 670 how a gear pump is to be designed for the delivery of thermally sensitive polymers, such as PVC; that is, in such a manner that the delivered medium forms the lubricating medium for the shaft bearings, and flow paths are provided for the delivered medium axially along the rotor shaft into the environment. However, the gear pump described there is not capable of delivering from a vacuum, thus from pressure conditions which make the advancing of the delivered medium as the lubricating medium seem difficult.
It is now recognized according to the invention that the gear pump which is known in principle from the Swiss Patent Document CH-PS-573 053, and which is normally used for the delivery of thinly to medium-viscous media from a reactor bulb under a vacuum, will also be easily suitable for the delivery of very contaminated thermoplastic material when its rotors, according to a gear pump normally used between the extruder and the extrusion tool, are disposed according to European Patent Document EP-A-0 189 670, although it is charged from a vacuum.
In the case of a preferred embodiment of the treatment stage according to the invention, the latter gear pump is used which is also inventive.